Functionalized oligothiophene-based heterocyclic aromatic fluorescent compounds with various donor-acceptor spacers and adjustable electronic properties: A theoretical and experimental perspective

Article abstract of DOI:10.1016/j.tet.2013.06.087

Syntheses, characterizations, optical, electrochemical, and thermal properties of a family of linear oligothiophene-based heterocyclic aromatic fluorescent compounds are described herein. They all have effective π-conjugated systems with D-π-D or A-π-A st

Full text of DOI:10.1016/j.tet.2013.06.087

Tetrahedron 69 (2013) 7290e7299
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Functionalized oligothiophene-based heterocyclic aromatic
fluorescent compounds with various donoreacceptor spacers and
adjustable electronic properties: a theoretical and experimental
perspective
,
,
Tao Tao a, Hui-Fen Qian a b, Kun Zhang a, Jiao Geng a, Wei Huang a
*
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing, Jiangsu Province, 210093, PR China
b College of Sciences, Nanjing University of Technology, Nanjing, Jiangsu Province, 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 April 2013
Received in revised form 4 June 2013
Accepted 21 June 2013
Available online 29 June 2013
Syntheses, characterizations, optical, electrochemical, and thermal properties of a family of linear
oligothiophene-based heterocyclic aromatic fluorescent compounds are described herein. They all have
effective p-conjugated systems with DepeD or AepeA structures as well as different bromo, thiocyano,
formyl, and triphenylamino tails. X-ray single-crystal structure of 2TPA2T methanol semisolvate reveals
a trans configuration with different dihedral angles between adjacent aromatic heterocycles. Theoretical
and experimental studies have been made to reveal the differences from related compounds with ad-
justable electronic properties. The influences of introducing different D/A functionalized tails on the
band-gap convergence have also been discussed, where the convergence behavior corrected via the
thienyl ring coefficient (ncorr) shows better correlation of linear fitting based on the extrapolation of
Keywords:
Oligothiophenes
Syntheses
Photophysics
Electrochemistry
DFT study
HOMOꢀLUMO gaps at the B3LYP/6-31G
*
level.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
efforts1,9 have been made on the studies of a variety of linear
conjugated oligomers. From the basic research perspective, explo-
ration of the convergence behavior9,10 is helpful for understanding
the differences from short conjugated oligomers to polymers
whose properties are calculated by the extrapolation of oligomeric
properties to infinite chain lengths in theory. Meanwhile, an ef-
fective strategy for designing experimentally low band-gap oligo-
mers is to use the miscellaneous conjugated donoreacceptor (DeA)
concept because different DeA units in the linear aromatic het-
erocyclic semiconducting compounds show different band gaps.11
On the one hand, the so-called core-embedded method12 can be
finely tuned from the structure to the property, such as size, sym-
metry, conformation, coordination site, dihedral angle between
adjacent aromatic heterocycles, the HOMOeLUMO gap, linear ab-
sorbance, fluorescence and electrochemical properties, solubility
and reaction activity by introducing different alkyl substituted
groups. For instance, core-substituted naphthalene diimides with
Research on the
p
-functional materials,1 such as thiophene-
based and fluorene-based molecules and polymers, has been
attracting considerable interests in view of their wide applications
on organic semiconducting materials and electronic devices, such
as dye-sensitized solar cells (DSSCs),2 organic solar cells (OSCs),3
organic light-emitting diodes (OLEDs),4 organic field-effect tran-
sistors (OFETs),5 chemosensors,6 biosensors,7 and electrochromic
devices.8 In particular, linear conjugated oligomers9 share many of
the properties of conjugated polymers with certain advantages
including their well-defined chemical structures, monodispersity,
better solubility, easier purification, fewer defects, and the possi-
bility to introduce versatile functionalities. However, rational de-
sign and synthesis of extended heterocyclic aromatic systems for
constructing high-performance electronic materials are still very
important and challenging issues.
Limited by the tremendous gap that still exists between the
structures and the properties, these problems have not been
completely solved. Up till now, many theoretical and experimental
tunable emission wavelengths are reported for fluorescence reso-
13
€
nance energy transfer studies by Wurthner et al. Namely, the
alkoxy-substituted derivatives are yellow dyes with green emission
and low photoluminescence quantum yields, whereas the amine-
substituted derivatives exhibit a color range from red to blue
with strong photoluminescence quantum yields up to 76%. On the
* Corresponding author. E-mail address: whuang@nju.edu.cn (W. Huang).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.06.087

T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
7291
other hand, the introduction of a functional terminal group14 into
oligothiophenes provides an opportunity for incorporating the
conjugated system covalently into a more complex system. For
example, -conjugated oligomers, based on benzothiadiazole and
thiophene, increasing and decreasing the donor and acceptor
strengths, are reported for nonlinear optics and near-IR emission by
the Reynolds’ group.15 For many applications, such as OLEDs and
photovoltaics, the HOMOeLUMO gap is critical to an optimized
device because it determines the color of the light emitted for an
OLED or the monochromatic incident photon-to-electron conver-
sion efficiency for a photovoltaic device.
number of aromatic heterocycles and the electronic and fluorescent
spectra, band-gap alterations, the limitation of solubility of com-
pounds, which is also influenced by the substituent effects. So
theoretical and experimental studies have been made to reveal the
differences from 42 related compounds (Scheme 2) with adjustable
electronic properties, where the influences of introducing seven
types of functionalized terminals (i.e., proton, bromo, imidazolyl,
pyridyl, thiocyano, formyl, and triphenylamino groups) and the
number of aromatic heterocycles (n¼1e6) on the convergence
behavior of HOMOeLUMO gaps have been included. It is found that
convergence behavior corrected via the thienyl ring coefficient
(ncorr) shows better correlation of linear fitting, which is based on
p-
p
Taking all the above-mentioned factors into consideration, it is
believed that the linear oligothiophene-based heterocyclic aro-
the extrapolation of HOMOꢀLUMO gaps at the B3LYP/6-31G
*
level.
matic fluorescent compounds, bearing delocalized
p-systems,
versatile DeA hybrids, and functionalized terminal groups, are
good candidates for the investigations on the design and synthesis
2. Results and discussion
of
p-conjugated oligomers from a theoretical and experimental
2.1. Syntheses and spectral characterizations
perspective. In our previous work, a family of 1,10-phenanthroline-
and bithiazole-based heterocyclic aromatic compounds with ter-
minal thienyl, imidazolyl, pyridyl groups has been described.16
Furthermore, temperature-dependent semiconducting and photo-
responsive properties of self-assembled nanocomposite films and
nanodevices fabricated from these compounds and their metal
complexes have been explored.17 Herein we have extended our
work on the oligothiophene-centered semiconducting and fluo-
rescent compounds where terminal bromo, thiocyano, formyl, and
triphenylamino groups are successfully introduced (Scheme 1). The
syntheses and full characterizations of this family of linear het-
erocyclic aromatic compounds with various electron-donating and
electron-withdrawing tails have been described, and the aim of
combining various donor and acceptor substituents into molecules
is to finely tune their electronic structures and compare their
spectroscopic, electrochemical, and thermal properties.
As shown in Scheme 2, 42 model compounds were involved in
the systematic comparisons for a theoretical and experimental
perspective in terms of band-gap engineering. Among them, 31 of
them are known compounds including 7 new compounds in this
work and the other 11 compounds are unreported. Our synthetic
strategy was based on the routes shown in Scheme 1, in which the
starting materials 3,300-dimethyl-2,20:50,200-terthiophene (3T-Me)
and 3,3000-dimethyl-2,20:50,200:500,2000-quaterthiophene (4T-Me) were
prepared according to our previously reported approaches.16c All the
target compounds were prepared by carbonecarbon (CeC) bond
and/or carbonenitrogen (CeN) bond cross-coupling reactions. The
combination of different cross-coupling methods, such as Kuma-
daeCorriu, SuzukieMiyaura, Ullmann reactions, have been carried
out to optimize the experimental conditions in order to prepare the
linear p-conjugated compounds with high yields. As can been seen
Scheme 1. Synthetic route of oligothiophene-based heterocyclic aromatic fluorescent compounds.
All these obtained linear heterocyclic aromatic compounds
promote us to further explore possible rules between their struc-
tures and properties, for example, the relationship between the
in Figs. SI1e9, all the heterocyclic aromatic compounds have been
characterized by1H,13C NMR, and EI-TOF-MSspectra, and the results
clearly demonstrate the formation of expected compounds.

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T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
Scheme 2. Molecular structures of oligothiophene-based heterocyclic aromatic fluorescent compounds.
In comparison with the oligothiophenes bearing no substituent,
we have previously demonstrated that the introduction of
-methyl group to the thiophene ring can effectively increase the
As
anticipated,
triphenylamino-terminated
compounds
2TPA2T, 2TPA3T-Me, and 2TPA4T-Me were synthesized with
satisfactory yields in the presence of a [Pd(PPh3)4] catalyst by
using a strong cross-coupling reagent, i.e., 4-(diphenylamino)
phenylboronic acid. Furthermore, the resultant yields for 2TPA2T,
2TPA3T-Me, and 2TPA4T-Me can be improved by using Cs2CO3
instead of Na2CO3 as a base. As shown in Scheme 1, formyl ter-
minated quaterthiophene compound 2CHO4T-Me was also firstly
prepared by the classical VilsmeiereHaack reaction in a yield of
71.3%, in which the mono-formyl compound CHO4T-Me was
carefully separated as a by-product in a yield of 3.8% by silica gel
column chromatography.
a
b
solubility of resulting compounds in organic solvents without
impacting the molecular planarity and electronic structure, which
facilitates the related CeC bond and CeN bond cross-coupling re-
actions and improves the final yields.16c In this work,
b-methyl-
thiophene extended quaterthiophene compounds 6T-Me and Br5T-
Me were firstly prepared in one-pot reaction (63.7 and 8.5%) by the
treatment of Grignard reagents obtained from corresponding
oligothiophene-based bromides and magnesium turnings with
a [NiCl2(dppp)] catalyst in dry THF. It is worth mentioning that
Br5T-Me has been further confirmed by the 1He1H correlation
spectroscopy (COSY) NMR shown in Fig. SI5c.
As illustrated in Fig. 1, all new compounds show characteristic
absorptions at 350e432 nm in their electronic spectra
Fig. 1. UVevis absorption spectra (a, b), fluorescence emission excited at 350 nm (c, d) and their visual photographs (e) for heterocyclic aromatic compounds in their methanol
solutions at room temperature with the same concentration of 5.0ꢁ10ꢀ5 mol/L.

T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
7293
corresponding to the
pep transitions within the conjugated sys-
*
tem of the whole aromatic heterocycles. As expected, the maximum
absorption wavelengths are shifted to lower energy bands when the
number of aromatic heterocycles is increased, which are compara-
ble with those in oligothiophene compounds. Moreover, it is noted
that this family of linear heterocyclic aromatic compounds are
fluorescence active, and compound Br5T-Me shows a yellow-green
fluorescence peak at 485 nm and a UVevis absorption peak at
395 nm (
3
¼16,300) simultaneously. Similar bathochromic shifts
have been found in the fluorescence emission spectra of compounds
2SCN4T-Me, 2SCN9T-Bu, 2TPA2T, 2TPA3T-Me, and 2TPA4T-Me
when the number of aromatic heterocycles (namely delocalized p-
systems) is increased. In contrast, compounds CHO4T-Me and
2CHO4T-Me have the poor intensity of fluorescence emission be-
cause of the fluorescence quenching of formyl group. Compared
with the 1,10-phenanthroline-based16c aromatic heterocyclic com-
pounds showing blue fluorescence emissions, obvious red-shifts
are observed for these oligothiophene-based compounds espe-
cially for those pyridyl and triphenylamino-terminated terthio-
phene and quaterthiophene compounds, where green fluorescence
emissions are observed. The alterations of molecular rigidity, pla-
narity, and electronic structures of oligothiophene-based com-
pounds are suggested to be the main reason for the above-
mentioned red-shifts in UVevis absorption and fluorescence
emission spectra.
Fig. 3. TGA graphs of oligothiophene-based compounds 2TPA2T, 2TPA3T-Me, 2TPA4T-
Me, 6T-Me, 2Im3T-Me, 2Im4T-Me, 2Py3T-Me, and 2Py4T-Me.
derivatives 2Im4T-Me, 2Py4T-Me, and 6T-Me show more re-
markable performance in the thermal stability compared with
corresponding terthiophene-based counterparts. In contrast, the
Td10 values for compounds 2TPA4T-Me, 2Br6T-Me, 2CHO4T-Me,
and 2SCN4T-Me are found to be in the range 217e289 ꢂC, in-
dicative of lower thermal stability.
2.2. Single-crystal structure of (2TPA2T)2$(CH3OH)
X-ray single-crystal diffraction study for compound 2TPA2T
methanol semisolvate reveals that the two central thiophene rings
are coplanar and they exhibit a trans configuration with the di-
hedral angle of 25.8(5)ꢂ between each central thiophene ring and
its neighboring benzene ring of the triphenylamino unit, as
depicted in Fig. 2. It is generally believed that increasing the pla-
narity, which could reduce the reorganization energy and enhance
the electronic coupling between adjacent molecules, would be
much more efficient than distorted stacking for the transportation
of charge carriers.18
2.4. Electrochemical properties
The electrochemical behavior of heterocyclic aromatic com-
pounds was examined by cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) in their 1.0ꢁ10ꢀ3 M CH2Cl2 solutions
containing 0.1 M TBAClO4 as the supporting electrolyte at different
scan rates (10, 20, 50, 80, 100 mV sꢀ1) in Fig. 4. All potentials re-
ported herein were calibrated with the ferrocene/ferrocenium
couple (Fcþ/Fc) as internal standard. Oxidation onset potentials
(Eooxnset), as well as HOMO/LUMO energy levels were determined by
a combined analysis of the CV, DPV and absorption data are sum-
marized in Table 1. The onset oxidation was measured relative to
the Fcþ/Fc couple for which an energy level of ꢀ5.10 eV versus
vacuum was assumed.20
As depicted in Fig. 4 and SI11, TPA-terminated oligothiophenes
show reversible and well-defined redox response in the CV mea-
surements. The onset oxidation potentials (Eoonxset) of these com-
pounds were determined to be 0.05 V for 2TPA4T-Me, 0.08 V for
2TPA3T-Me, and 0.14 V for 2TPA2T. In general, the HOMO of DeA
oligomer is determined by the HOMO of the donor unit, while the
LUMO of DeA oligomer is controlled by the LUMO of acceptor unit.
As can be seen in Fig. 4, one or two oxidation waves have been
observed for all compounds, and the reduction potentials as well as
the measured currents are found to be dependent on their molec-
ular structures. The first oxidation wave can be attributed to the
formation of cation-radical species and the second one is ascribed
to the successive oxidation of cation-radical to its corresponding
dication, which have been observed previously in other oligothio-
phene systems.20,21
Fig. 2. An ORTEP diagram (30% thermal probability ellipsoids) of the molecular
structure of 2TPA2T showing the dihedral angles and relative configurations between
adjacent aromatic heterocycles.
2.3. Thermal stability
Thermal durability has significant impact on the device per-
formance, which is one of the most essential parameters for fine-
tuned applications of the small organic molecules.19 Twelve
oligothiophene-based compounds have been checked herein in
TGA measurements and a parameter of Td10 (10% weight-loss
temperature) is used to describe the thermal stability of com-
pounds. As shown in Fig. 3, TGA of oligothiophenes 2TPA2T,
2TPA3T-Me, 2TPA4T-Me, 6T-Me, 2Im3T-Me, 2Im4T-Me, 2Py3T-
Me, and 2Py4T-Me indicates that they can keep unchangeable
when the temperature is below 200 ꢂC. The Td10 values for
them range from 271 to 451 ꢂC. Furthermore, quaterthiophene
2.5. Density function theory (DFT) computations
DFT calculations are carried out with the Gaussian 03, Revision
C.02 programs,22 using the B3LYP method and 6-31G
basis set.
*
The fixed atom coordinates of 42 oligothiophene-based com-
pounds, based on the structural parameters determined by the

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T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
Fig. 4. CV (a, b, c, d) and DPV (e, f) for heterocyclic aromatic compounds in CH2Cl2 (1.0ꢁ10ꢀ3 M) containing 0.10 M of TBAClO4 under argon. The different scanning rate: 10, 20, 50,
80, 100 mV/s (a) 2TPA2T; (b) 2TPA3T-Me; (c) 2TPA4T-Me; (d) 2SCN4T-Me versus Fcþ/Fc.
Table 1
UVevis absorption and fluorescence emission data, optical, electrochemistry, and calculated HOMOeLUMO energy gaps (Eg) for related heterocyclic aromatic compounds
(L molꢀ1 cmꢀ1
)
Eogpta (eV)
Ecgalcdb (eV) Fluorescence lmax (nm) Td10 (ꢂC)
Eoonxsetd (V) EHOMO (eV) ELUMO (eV)
c
e
f
Compounds
UVevis lmax [nm (eV)]
3
2TPA2T
409 (3.03)
414 (2.99)
357 (3.47)
399 (3.11)
430 (2.88)
389 (3.19)
420 (2.95)
419 (2.96)
410 (3.02)
384 (3.23)
390 (3.18)
432 (2.87)
395 (3.14)
411 (3.02)
392 (3.16)
3200
2.61
2.54
2.96
2.66
2.46
2.73
2.54
2.47
2.54
2.79
2.71
2.39
2.64
2.57
2.74
3.40
3.07
3.45
2.99
2.83
3.02
2.80
2.78
2.83
3.21
2.97
2.50
2.83
2.67
2.61
482
508
460
506
525
452
533
454
584
482
468
547
485
503
513
432
398
335
273
271
358
451
217
d
0.14
0.08
0.35
d
ꢀ5.24
ꢀ5.18
ꢀ5.45
d
ꢀ2.63
ꢀ2.64
ꢀ2.49
d
2TPA3T-Me
2Im3T-Me
2Py3T-Me
2TPA4T-Me
2Im4T-Me
2Py4T-Me
2CHO4T-Me
CHO4T-Me
2Br4T-Me
2SCN4T-Me
2SCN9T-Bu
Br5T-Me
44,500
47,700
45,800
30,600
57,700
27,800
5100
27,100
32,700
3580
0.05
d
ꢀ5.15
d
ꢀ2.69
d
0.30
d
ꢀ5.40
d
ꢀ2.86
d
d
d
d
d
d
d
d
289
d
0.55
0.14
0.25
0.18
d
ꢀ5.65
ꢀ5.24
ꢀ5.35
ꢀ5.28
d
ꢀ2.94
ꢀ2.85
ꢀ2.71
ꢀ2.71
d
62,900
16,300
6060
278
417
220
6T-Me
2Br6T-Me
22,000
a
Optical band gap determined from the UVevis absorptions in their methanol solutions.
b
c
d
e
f
The geometries are calculated by B3LYP method and 6-31G
10% Weight-loss temperature.
*
basis set.
Oxidation onset potentials determined from DPV versus Fcþ/Fc.
onset
Calculated from EHOMO¼ꢀ(E
þ5.10).
ox
Calculated from ELUMO¼EHOMOþEgopt
.

T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
7295
X-ray diffraction method and fully optimization, are used for the
highest occupied molecular orbital (HOMO) and the lowest un-
occupied molecular orbital (LUMO) gap calculations (Table 1). The
resultant HOMOeLUMO gaps for TPA-extended compounds
2TPA2T, 2TPA3T-Me, and 2TPA4T-Me are 3.40, 3.07, and 2.83 eV,
respectively, which are analogous to their UVevis absorption
peaks.
According to the DPV measurements, the corresponding oxi-
dation potentials for four quaterthiophene-based derivatives
2SCN4T-Me, 2Py4T-Me,16c 2TPA4T-Me, and 6T-Me were de-
termined to be 0.55, 0.30, 0.05, and 0.18 V, respectively (Table 1,
Fig. 4e and f). Further analyses of the frontier orbitals reveal that
the introduction of electron-donating groups raises the HOMO
energy levels, while the introduction of electron-withdrawing
groups lowers the LUMO energy levels, which will greatly de-
crease the HOMOeLUMO gaps of resultant compounds. It is noted
that there is an almost linear relationship between the experi-
mentally determined (CV and UVevis absorption spectra) and the
theoretically calculated energy levels for the HOMOs and LUMOs
of these triphenylamino- and thiocyano-terminated oligothio-
phene hybrids (Figs. 5 and 6). This relationship demonstrates that,
despite the uncertainty in the calculated absolute values, theo-
retical calculations can serve as a useful tool to predict and guide
the synthesis of future oligomer and polymer materials at least in
some cases.
Fig. 6. (a) HOMO (bottom) and LUMO (top) of 2SCN4T-Me (left) and 2SCN9T-Bu (right)
calculated with B3LYP/6-31G
*
. (b) Energy level correlation between CV and compu-
tational data; Y-axis: 2SCN4T-Me (red) and 2SCN9T-Bu (green).
Fig. 5. HOMOs and LUMOs of 2TPA2T (a), 2TPA3T-Me (b), and 2TPA4T-Me (c) calculated with B3LYP/6-31G
and computational data; Y-axis: 2TPA2T (red), 2TPA3T-Me (green), 2TPA4T-Me (blue).
*
. (d) Energy level correlation between CV, UVevis absorption spectra,
To make further comparisons of the HOMOeLUMO gaps from
a theoretical and experimental perspective, the extrapolation for
oligothiophenes and chain length correlates linearly with an em-
pirically obtained value of 1/(n0.75). The same correction strategy
has been used herein, but a new thienyl ring coefficient (ncorr) has
been introduced (Eq. 1) to correct the effective number of oligo-
thiophene (neff) according to Kuhn’s equation23 (see SI) due to the
presence of different terminal groups. So the neff coefficient in-
cludes the contribution of different terminal groups in our
a series of
p
-conjugated oligothiophenes at the B3LYP/6-31G level
*
was carried out. However, the theoretical prediction is not
accurate in accordance with the experimental results. To solve this
problem, Zade et al.10 used the correction index method where the
relationship between the first adiabatic ionization potential of

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T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
compounds, which leads to better correlation of linear fitting. For
example, the prediction of HOMOeLUMO gap for a nonathiophene
compound 2SCN9T-Bu is 2.63 eV from the curve without corrected
(Fig. SI17c, Y¼2.22Xþ2.38), while the prediction value is 2.45 eV
with the neff coefficient correction (Fig. SI17d, Y¼4.41Xþ2.00).
Compared where the optical band gap and HOMOeLUMO gap for
2SCN9T-Bu (Eogpt¼2.39 and Ecgalcd¼2.50 eV),17 it is suggested that the
introduction of the neff coefficient can help to predict more accu-
rately the HOMOeLUMO gaps of various DeA spacer functionalized
oligothiophene systems. Similar results have been obtained for all
the oligothiophene-based compounds with different end-capped
groups, as can be seen in Fig. SI17.
Y ¼ A=neff þ B
(1)
neff ¼ n þ ncorr
(2)
On the one hand, by introducing a parameter of ncorr, the
convergence behavior shows not only better correlation of linear
fitting, but also more precise theoretical prediction of band gap
for oligomers and polymers. Actually, another important question
is whether the corrected value is reasonable. We believe the an-
swer is yes at least for polythiophenes because the limitation of
band gap is around 2.00 eV based on the extrapolation of
HOMOeLUMO gaps (see Fig. SI17). On the other hand, these pa-
rameters can assist us to evaluate the effects of different DeA
terminal groups from the quantitative viewpoint. As illustrated in
Fig. 7, the contour diagrams of compounds mentioned in this
work have the traits of visualizing, intuition, and easy un-
derstanding, which can guide us to rationally design the HOMO
and LUMO energy levels and finely tune the band gaps for these
oligothiophene-based compounds. More importantly, this
method can help us to find some potentially useful materials with
required band gaps and suitable energy levels more easily, hence
improving our ways of working and reducing drastically the re-
search time.
Based upon the above-mentioned electronic spectra, electro-
chemistry, and DFT computational data, systematic comparisons in
terms of the band-gap engineering show that the energy discrep-
ancy between the HOMO and LUMO decreases as the length of
conjugation increases. However, when the number of monomer
units exceeds a certain value at which the effective conjugation
length is saturated, the band gap starts to level off. Therefore, un-
limited extension of the conjugation length results only in a limited
reduction of the band gap. Moreover, incorporation of the electron-
donating or electron-withdrawing substituents directly into the
aromatic unit in the main chain represents another effective way to
finely tune the electronic structure by perturbing the molecular
orbitals through either inductive or mesomeric effects. We think
our effects on studying the functionalized oligothiophene-based
heterocyclic aromatic fluorescent compounds with various DeA
spacers and adjustable electronic properties can provide a theo-
retical screening approach for the discovery of novel organic
semiconductor cores and terminals before conventional chemical
synthesis.
Fig. 7. Contour diagrams of HOMO (a), LUMO (b) energy level, and HOMOeLUMO gap
(c) for 42 oligothiophene-based compounds calculated with B3LYP/6-31G
*
.
and spectral comparisons have been carried out for these mono-
and bi- -methylthiophene/thiocyanato/triphenylamino/formyl
b
extended oligothiophene compounds with different number of
aromatic heterocycles in order to reveal the differences between
cross-coupling approaches and experimental conditions on the
CeC bond and CeN bond formation, molecular conformation, and
dihedral angles between neighboring heterocycles, band gaps and
energy levels, electronic, fluorescent, and electrochemistry spectra
of related compounds. X-ray single-crystal structure of 2TPA2T
methanol semisolvate reveals a trans configuration with different
dihedral angles between adjacent aromatic heterocycles. Com-
pounds 2TPA2T, 2Py4T-Me, and 6T-Me show excellent perfor-
mance in the thermal stability, which suggests that they may be
good candidates for the device fabrication.
3. Conclusions
Following the idea of introducing specific organic DeA hybrids
to promote the direct electronic separation and transportation and
retaining the effective and linear delocalized
p systems simulta-
neously, a series of linear oligothiophene-based heterocyclic aro-
matic fluorescent compounds with molecular lengths more than
2 nm (2TPA2T, 2TPA3T-Me, 2TPA4T-Me, 2SCN4T-Me, 2SCN9T-Bu,
2CHO4T-Me, CHO4T-Me, 6T-Me, Br5T, and 2Br6T) have been re-
ported in this work. Synthetic, structural, thermal, computational,

T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
7297
A theoretical and experimental perspective has been carried out
for this family of linear oligothiophene-based heterocyclic aromatic
fluorescent compounds, which have effective -conjugated sys-
tems with De eD or Ae eA structures and different terminal
J¼7.5 Hz, phþthio), 7.07 (d, 4H, J¼8.3 Hz, ph), 7.04 (t, 4H, ph). 13
C
NMR (125 MHz, CDCl3)
d
: 147.5, 147.4, 142.9, 136.0, 129.3, 128.1,
p
126.4, 124.6, 124.3, 123.6, 123.2, 122.9. EI-TOF-MS (m/z): calcd for
[C44H32N2S2]þ: 652.2 (100.0%), 653.2 (49.9%). Found: 652.0
(100.0%), 653.0 (11.6%).
p
p
groups namely, bromo, imidazolyl, pyridyl, thiocyano, formyl, and
triphenylamino tails. The influences of introducing different DeA
functionalized tails on the band-gap convergence have also been
discussed, where the convergence behavior corrected via the
thienyl ring coefficient shows better correlation of linear fitting
based on the extrapolation of HOMOeLUMO gaps. As we know,
designing better organic optoelectronic materials requires a com-
prehensive understanding of the electronic structure. A theoretical
screening and optimizing approach can help us find novel organic
semiconductors as soon and as effectively as possible prior to the
chemical synthesis. We hope this study can provide some useful
information on the general rules between structures and proper-
ties. Further studies are being undertaken on the field-effect, light-
emitting, photoresponsive, and photovoltaic properties of these
linear aromatic heterocycle-based nanowires, nanocomposite
films, and nanodevices.
4.2.2. 2TPA3T-Me. The synthesis of 2TPA3T-Me was similar to that
described for 2TPA2T. Yield: 79.5% (0.61 g) as an orange solid.
Melting point: >300 ꢂC. Anal. Calcd for [C50H38N2S3]: C, 78.70; H,
5.02; N, 3.67%. Found: C, 78.58; H, 5.12; N, 3.55%. Main FTIR ab-
sorptions (KBr pellets, cmꢀ1): 3457 (b), 2365 (m), 1587 (s), 1494
(vs), 1324 (m), 1273 (s), 1175 (w), 1087 (m), 1025 (m), 813 (m), 749
(m), 694 (m), 509 (w). 1H NMR (500 MHz, CDCl3)
d: 7.45 (d, 4H,
J¼8.0 Hz, ph), 7.29 (t, 8H, ph), 7.14 (d, 8H, J¼8.0 Hz, ph), 7.11 (m, 4H,
thio), 7.05 (m, 8H, ph), 2.43 (s, 6H, CH3). 13C NMR (125 MHz, CDCl3)
d
: 146.7, 141.4, 134.7, 129.3, 124.6, 124.3, 124.2, 124.1, 123.6, 123.1,
122.8, 122.7, 15.9. EI-TOF-MS (m/z): calcd for [C50H38N2S3]þ: 762.2
(100.0%), 763.2 (57.2%), 764.2 (15.3%). Found: 762.1 (100.0%), 763.2
(38.1%), 764.2 (6.7%).
4.2.3. 2TPA4T-Me. The synthesis of 2TPA4T-Me was similar to that
described for 2TPA2T. Yield: 75.3% (0.64 g) as a red solid. Melting
point: 219e221 ꢂC. Anal. Calcd for [C54H40N2S4]: C, 76.74; H, 4.77; N,
3.31%. Found: C, 76.52; H, 4.93; N, 3.15%. Main FTIR absorptions (KBr
pellets, cmꢀ1): 3451 (b), 2359 (m), 1587 (s), 1489 (vs), 1324 (m),
1267 (s), 1092 (s), 1020 (s), 798 (s), 757 (m), 695 (s), 618 (w), 510
4. Experimental section
4.1. General
All melting points were measured without any corrections.
Unless otherwise specified, solvent of analytical grade was pur-
chased directly from commercial sources and used without any
further purification. Tetrahydrofuran (THF) was freshly distilled
from the sodium/benzophenone mixture prior to use. Anhydrous
solvents were drawn into syringes under the flow of dry N2 gas
and directly transferred into the reaction flasks to avoid contami-
nation. Column chromatography was carried out on silica gel
(300e400 mesh) and analytical thin-layer chromatography (TLC)
was performed on glass plates of silica gel GF-254 with detection by
UV. Standard techniques for synthesis were carried out under argon
atmosphere. Heterocyclic aromatic compounds 3,300-dimethyl-2,20:
50,200-terthiophene (3T-Me), 5,500-dibromo-3,300-dimethyl-2,20:50,
200-terthiophene (2Br3T-Me), 3,3000-dimethyl-2,20:50,200:500,2000-qua-
terthiophene (4T-Me), and 5,5000-dibromo-3,3000-dimethyl-2,20:50,200:
500,2000-quaterthiophene (2Br4T-Me) were prepared from 2,5-
dibromothiophene (2BrT) and 5,50-dibromo-2,20-bithiophene (2Br
2T) via literature methods.16c
(w). 1H NMR (500 MHz, CDCl3)
8H, ph), 7.14 (d, 12H, J¼7.65 Hz, phþthio), 7.05 (m, 10H, phþthio),
2.44 (s, 6H, CH3). 13C NMR (125 MHz, CDCl3)
: 147.4, 141.5, 135.0,
d: 7.45 (d, 4H, J¼8.6 Hz, ph), 7.27 (t,
d
129.3, 125.6, 124.6, 124.3, 124.1, 123.9, 123.5, 123.2, 122.8, 15.9. EI-
TOF-MS (m/z): calcd for [C54H40N2S4]þ: 844.2 (100.0%), 845.2
(62.1%), 846.2 (19.1%). Found: 844.2 (100.0%), 845.2 (32.4%), 846.2
(8.9%).
4.2.4. 6T-Me and Br5T-Me. Activated Mg turnings (1.04 g,
42.95 mmol) in 20 mL of anhydrous THF and a catalytic amount of
iodine were added to a flame-dried 50 mL three-neck flask
equipped with vigorous stirring at the room temperature under
argon atmosphere. Then a solution of 2-bromo-3-methylthiophene
(2.5 mL, 22.20 mmol) in 10 mL of anhydrous THF was slowly
dropped into the reaction mixture. Once the vigorous reaction had
started, the rest of the 2-bromo-3-methylthiophene solution was
added dropwise to keep the mixture refluxing over 20 min. The
reaction mixture was allowed to proceed for 2 h at the room
temperature and cannulated into an ice-cooled solution of 2Br4T-
Me (2.58 g, 5.00 mmol) and [Ni(dppp)Cl2] (0.11 g, 0.20 mmol) in dry
THF (40 mL). After being stirred for 2 h at the room temperature,
the reaction mixture was refluxed for another 12 h and then cooled
to the room temperature. The reaction mixture was quenched with
saturated NH4Cl aqueous solution, extracted thoroughly with
chloroform (CHCl3) until no more products could be detected by
TLC, washed with brine. The resulting organic layer was dried over
anhydrous sodium sulfate and filtered. Compounds 6T-Me and
Br5T-Me were purified by silica gel column chromatography
employing CHCl3 solution to give red solid in yields of 1.75 g (63.7%)
for 6T-Me and 0.23 g (8.5%) for Br5T-Me, respectively. Compound
6T-Me: Melting point: >300 ꢂC. Anal. Calcd for [C28H22S6]: C, 61.05;
H, 4.03%. Found: C, 60.87; H, 4.21%. Main FTIR absorptions (KBr
pellets, cmꢀ1): 3408 (b), 2920 (s), 2854 (m), 1446 (vs), 1062 (m),
1022 (m), 823 (s), 779 (s), 709 (s), 615 (m). 1H NMR (500 MHz,
4.2. Syntheses and characterizations of heterocyclic aromatic
compounds
4.2.1. 2TPA2T. A mixture of 2Br2T (0.32 g, 1.00 mmol), 4-(diphe-
nylamino)phenylboronic acid (1.16 g, 4.00 mmol), cesium carbon-
ate (1.30 g, 4.00 mmol), [Pd(PPh3)4] (0.06 g, 0.05 mmol), toluene
(20 mL), and water (5 mL) was degassed for 0.5 h and heated to
reflux for 40 h under argon atmosphere. The mixture was then
allowed to cool to the room temperature and extracted with
chloroform. The resulting organic layer was dried over anhydrous
sodium sulfate and filtered. The filtrate was evaporated, and the
residue was purified by column chromatography over silica gel
using hexane and dichloromethane (1:1) as eluent to give 0.57 g
(88.1%) of 2TPA2T as a yellow solid. The yellow single crystals of
2TPA2T suitable for X-ray diffraction measurement were grown
and isolated from DMF by slow evaporation in air at room tem-
perature for 2 weeks. Melting point: >300 ꢂC. Anal. Calcd for
[C44H32N2S2]: C, 80.95; H, 4.94; N, 4.29%. Found: C, 80.71; H, 5.12; N,
4.13%. Main FTIR absorptions (KBr pellets, cmꢀ1): 3450 (b), 2361
(m), 1590 (s), 1490 (vs), 1327 (m), 1283 (s), 1183 (w), 1070 (w), 830
(w), 796 (w), 757 (m), 694 (m), 512 (w). 1H NMR (500 MHz, CDCl3)
CDCl3)
(s, 2H, thio), 6.88 (d, 2H, J¼5.0 Hz, thio), 2.44 (d,12H, J¼5.4 Hz, CH3).
13C NMR (125 MHz, CDCl3)
: 136.3, 135.2, 134.4, 134.2, 134.0, 131.5,
d: 7.14 (d, 4H, J¼4.6 Hz, thio), 7.07 (d, 2H, J¼3.2 Hz, thio), 6.94
d
130.2, 129.5, 125.8, 123.9, 123.3, 29.5, 29.2, 15.6, 15.4. EI-TOF-MS
(m/z): calcd for [C28H22S6]þ: 550.0 (100.0%), 551.0 (30.5%), 552.0
(27.2%). Found: 549.8 (100.0%), 550.9 (52.8%), 551.9 (12.2%).
d: 7.46 (d, 4H, J¼8.5 Hz, ph), 7.28 (t, 8H, ph), 7.13 (d, 12H¼8Hþ4H,

7298
T. Tao et al. / Tetrahedron 69 (2013) 7290e7299
Compound Br5T-Me: Melting point: >300 ꢂC. Anal. Calcd for
[C23H17BrS5]: C, 51.77; H, 3.21%. Found: C, 51.55; H, 3.43%. Main FTIR
absorptions (KBr pellets, cmꢀ1): 3409 (b), 2922 (s), 2362 (w), 1730
(m), 1448 (s), 1062 (s), 788 (s), 710 (m), 618 (m). 1H NMR (500 MHz,
of 1.48 g (71.3%) for 2CHO4T-Me and 0.07 g (3.8%) for CHO4T-Me,
respectively. Compound 2CHO4T-Me: Melting point: 208e210 ꢂC.
Anal. Calcd for [C20H14O2S4]: C, 57.94; H, 3.40%. Found: C, 57.78; H,
3.62%. Main FTIR absorptions (KBr pellets, cmꢀ1): 3406 (vs), 2950
(m), 2212 (m), 1708 (m), 1581 (vs), 1350 (s), 1168 (m), 790 (m). 1H
CDCl3)
d
: 7.14 (d, 2H, J¼3.4 Hz, thio), 7.12 (d, 1H, J¼3.0 Hz, thio), 7.07
(d, 1H, J¼2.8 Hz, thio), 6.99 (d, 1H, J¼2.8 Hz, thio), 6.93 (s, 1H, thio),
NMR (500 MHz, CDCl3)
(d, 2H, J¼3.8 Hz, thio), 7.24 (d, 2H, J¼3.8 Hz, thio), 2.50 (s, 6H, CH3).
13C NMR (125 MHz, CDCl3)
: 182.4, 140.4, 139.9, 138.2, 135.0, 134.9,
d: 9.83 (s, 2H, CHO), 7.56 (s, 2H, thio), 7.27
6.88 (d, 1H, J¼4.9 Hz, thio), 6.86 (s, 1H, thio), 2.43 (d, 6H, J¼3.4 Hz,
CH3), 2.37 (s, 3H, CH3). 13C NMR (125 MHz, CDCl3)
d: 137.1, 135.7,
d
134.5, 134.2, 134.0, 132.4, 131.6, 130.4, 129.7, 126.5, 125.9, 124.1,
123.8, 123.3, 110.1, 15.7, 15.5, 15.4. EI-TOF-MS (m/z): calcd for
[C23H17BrS5]þ: 533.9 (100.0%), 531.9 (82.7%), 534.9 (28.5%), 532.9
(20.7%). Found: 533.9 (100.0%), 531.9 (97.2%), 534.9 (7.5%), 532.9
(9.6%).
128.1, 124.8, 29.6, 15.9. EI-TOF-MS (m/z): calcd for [C20H14O2S4]þ:
414.0 (100.0%), 415.0 (25.1%). Found: 413.9 (100.0%), 414.9 (1.46%).
Compound CHO4T-Me: Melting point: 170e172 ꢂC. Anal. Calcd for
[C19H14OS4]: C, 59.03; H, 3.65%. Found: C, 58.85; H, 3.83%. Main FTIR
absorptions (KBr pellets, cmꢀ1): 3446 (b), 1708 (m), 1657 (s), 1446
(m), 1377 (m), 1238 (m), 1184 (m), 1079 (w), 858 (w), 789 (m), 679
4.2.5. 2Br6T-Me. In the absence of light, NBS (0.39 g, 2.20 mmol)
was dissolved in DMF (5 mL) and injected into a solution of 6T-Me
(0.55 g, 1.00 mmol) in DMF (20 mL) at 40 ꢂC under argon atmo-
sphere. The mixture was stirred for 4 h at 60 ꢂC and then cooled to
the room temperature. Water (50 mL) was added into the mixture,
and the crude yellow solid was precipitated, filtered, and rinsed
with a 50% ethanol/water solution. The desired compound 2Br6T-
Me was purified by silica gel column chromatography (CHCl3) to
give red solid in yields of 0.67 g (94.3%). Melting point: >300 ꢂC.
Anal. Calcd for [C28H20Br2S6]: C, 47.46; H, 2.84%. Found: C, 47.22; H,
3.02%. Main FTIR absorptions (KBr pellets, cmꢀ1): 3406 (b), 2922 (s),
2854 (m), 1500 (m), 1446 (s), 1071 (m), 1018 (m), 829 (s), 788 (vs),
(m), 471 (w). 1H NMR (500 MHz, CDCl3)
d: 9.82 (s, 1H, CHO), 7.55 (s,
1H, thio), 7.25 (d, 1H, J¼4.1 Hz, thio), 7.18 (m, 3H, thio), 7.07 (d, 1H,
J¼3.6 Hz, thio), 6.91 (d, 1H, J¼5.0 Hz, thio), 2.49 (s, 3H, CH3), 2.44 (s,
3H, CH3). 13C NMR (125 MHz, CDCl3)
d: 182.4, 141.5, 140.4, 139.7,
139.1, 136.6, 135.8, 134.7, 134.4, 134.1, 131.6, 128.1, 126.2, 124.6, 124.1,
123.7, 29.7, 15.9, 15.5. EI-TOF-MS (m/z): calcd for [C22H16N2S2]þ:
386.6. Found: 386.0.
4.3. X-ray data collection and structural determination
Single-crystal sample of 2TPA2T methanol semisolvate was
covered with glue and mounted on a glass fiber and then used for
data collection. Crystallographic data were collected on a Bruker
612 (m), 460 (m). 1H NMR (500 MHz, CDCl3)
d
: 7.15 (d, 2H, J¼3.1 Hz,
thio), 7.07 (d, 2H, J¼3.9 Hz, thio), 6.87 (d, 4H, J¼9.4 Hz, thio), 2.43 (s,
6H, CH3), 2.36 (s, 6H, CH3). EI-TOF-MS (m/z): calcd for
[C28H20Br2S6]þ: 707.8 (100.0%), 709.8 (68.1%), 705.8 (45.1%), 708.8
(26.9%). Found: 707.8 (100.0%), 709.8 (30.7%), 705.8 (21.6%), 708.8
(11.2%).
SMART 1K CCD diffractometer, using graphite mono-chromated Mo
ꢀ
Ka
radiation (
l
¼0.71073 A). The crystal system was determined by
Laue symmetry and the space group was assigned on the basis of
systematic absences using XPREP.24 Absorption corrections were
performed to all data and the structure was solved by direct
methods and refined by full-matrix least-squares method on Fo2bs by
using the SHELXTL-PC software package.25 All non-H atoms were
anisotropically refined and all hydrogen atoms were inserted in the
calculated positions assigned fixed isotropic thermal parameters
and allowed to ride on their respective parent atoms. The summary
of the crystal data, experimental details, and refinement results is
listed in Table SI1, whereas bond distances and angles are given in
Table SI2.
4.2.6. 2SCN4T-Me. A solution of compound 4T-Me (0.66 g,
1.85 mmol) in CHCl3 (15 mL) was added dropwise to a mixture of
KSCN (10.90 g, 112.15 mmol) in methanol (30 mL) and Br2 (2.9 mL,
56.62 mmol) in CHCl3 (10 mL) at ꢀ78 ꢂC. This mixture was stirred
for 4 h at the room temperature, quenched with water, extracted
with CHCl3, and dried by anhydrous sodium sulfate. The solvent
was removed by a rotatory evaporator and the residue was purified
by silica gel column chromatography (CHCl3). A red powder was
obtained in a yield of 0.67 g (76.2%) after recrystallization from
CHCl3/n-hexane. Melting point: 275e278 ꢂC. Anal. Calcd for
[C20H12N2S6]: C, 50.82; H, 2.56; N, 5.93%. Found: C, 50.64; H, 2.82;
N, 5.75%. Main FTIR absorptions (KBr pellets, cmꢀ1): 3449 (b), 2359
(m), 2152 (m), 1644 (s), 1582 (m), 1438 (s), 1400 (m), 1099 (s), 986
Acknowledgements
This work was supported by the Major State Basic Research
Development Programs (Nos. 2013CB922101 and 2011CB933300),
the National Natural Science Foundation of China (Nos. 21171088
and 21021062) and Qing Lan Project.
(m), 855 (m), 780 (s). 1H NMR (500 MHz, CDCl3)
7.18 (d, 2H, J¼3.3 Hz, thio), 7.11 (d, 2H, J¼3.6 Hz, thio), 2.41 (s, 6H,
d: 7.22 (s, 2H, thio),
CH3). 13C NMR (125 MHz, CDCl3)
d: 141.5, 135.3, 133.9, 127.6, 124.6,
Supplementary data
115.2, 110.2, 15.5. EI-TOF-MS (m/z): calcd for [C20H12N2S6]þ: 471.9
(100.0%), 473.9 (27.4%), 472.9 (21.8%). Found: 471.9 (100.0%), 472.9
(4.5%), 473.9 (2.7%).
Tables of the crystal data, experimental details and refinement
results, selected bond lengths and angles, figures of
pep stacking
interactions, DFT computational details, 1H, 13C, 1He1H COSY NMR
and EI-TOF-MS spectra, and electrochemistry diagrams for related
compounds are included. Supplementary data associated with this
article can be found in the online version, at http://dx.doi.org/
10.1016/j.tet.2013.06.087.
4.2.7. 2CHO4T-Me and CHO4T-Me. To a three-necked round bot-
tom flask were added 4T-Me (1.79 g, 5.00 mmol), 7.7 mL
(100.00 mmol) of anhydrous DMF, and 40 mL of anhydrous chlo-
roform. The solution was cooled to 0 ꢂC, and 9.3 mL of POCl3
(100.00 mmol) was added dropwise with stirring. The solution was
then warmed to room temperature followed by refluxing for 4 h,
then cooled to room temperature and poured into ice water. The
water layer was extracted with dichloromethane (3ꢁ50 mL), and
the organic layers were combined, washed with water (2ꢁ50 mL)
and saturated aqueous sodium bicarbonate (2ꢁ50 mL), and then
dried by anhydrous sodium sulfate. After evaporation of the sol-
vent, compounds 2CHO4T-Me and CHO4T-Me were purified by
silica gel column chromatography (CHCl3) to give red solid in yields
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